1. Field of the Invention
The invention relates to the ablation of tissue during electrosurgical procedures. More particularly, the invention relates to tissue ablation by a monopolar electrosurgical device in a fluid environment during arthroscopic procedures.
2. Description of the Prior Art
Electrosurgical procedures are commonly performed in either a monopolar mode, using a probe having an active electrode placed adjacent tissue to be operated upon, with a return or common electrode placed externally on the patient's body, or a bipolar mode where both active and return electrodes are on the same probe. The procedures are being used to cut or coagulate tissue, these different functions accomplished by applying a different energy waveform and/or power level to the electrodes. Recently, bipolar electrosurgical devices have been developed for endoscopic tissue ablation at surgical sites filled with a conductive fluid rather than simply cutting or coagulation. Such new devices require special, dedicated and costly electrosurgical generators, new bipolar electrode designs and high power levels. The term "ablation" in the context of a surgical procedure is generally defined as the removal of tissue by vaporization. Ablation has the connotation of removing a relatively large volume of tissue. It can be stated that since ablation is the removal of tissue by high-density electrical discharge in a conductive fluid environment, ablation of a sort occurs from the edges of all electrosurgical electrodes used in a cutting mode. This effect, which is independent of whether the return path is provided by a conventional return pad (i.e. monopolar) or a return electrode immersed in the conductive fluid filled space (i.e. bipolar), is related to the volumetric ablation which is the subject of this invention. However, the ablative properties of the invention will be understood to be quite different from known devices.
It is well known to surgeons that for a given electrode design, higher power values give increased rates of tissue removal because the volume of tissue removed (during cutting, for example) is dependent on the power density at the active electrode. This applies to monopolar and bipolar devices. However, the power density required for tissue ablation has not generally been available over large enough surfaces in known monopolar electrodes and that is why surgeons desiring to perform electrosurgical volumetric ablation have had to use the aforementioned bipolar ablation systems.
Power density on the surface of a monopolar electrode is somewhat dependent on the conductivity of tissues or fluids in contact with the electrode. The conductive fluids used in electrosurgery are highly conductive and produce non-uniform current density at the electrode surface. Maximizing this power density over large enough surfaces facilitates tissue ablation, and the invention facilitates the proper power density over large enough surfaces at power levels lower than the aforementioned bipolar tissue ablation devices.
For an electrosurgical instrument working in a space filled with conductive fluid, such as during an arthroscopic procedure, current density is higher at the edges of the electrode than on its broader or flatter surfaces. When sufficient power is supplied, the current density at the edge of an electrode in this environment is sufficient to raise the temperature of the fluid thereby making it more conductive. The increased current flow due to this increased conductivity further raises the fluid temperature, which increases the conductivity, which increases the current flow, etc. This continues until the fluid at the electrode edge begins to form a gas phase due to boiling and a luminous discharge becomes visible due to localized arcing. It is believed that the high current density discharge and intense heat at the electrode edge actually perform the ablation. Similarly, bringing the edge of the instrument into contact or sufficiently close proximity with tissue will facilitate initiation of discharge from the edge of the electrode nearest the tissue. If sufficient power is supplied after such high-density discharge is initiated, the instrument can be withdrawn slightly from the tissue while maintaining the high-density discharge at the electrode edge. This phenomenon is well known to surgeons using conventional monopolar electrosurgical instruments.
Although all electrodes used in a cutting mode in a field filled with conductive fluid produce ablation at their edges, not all electrode shapes are equally useful for the removal of relatively large volumes of tissue by ablation. For example, conventional blade-like electrodes are poorly suited for the bulk ablation of tissue due to the small amount of edge area able to produce high density discharge. Similarly, solid cylindrical electrodes also have a small amount of edge area compared to non-edge area. The inherent inefficiency of these shapes necessitates very high power levels relative to the surface area. The efficiency of an electrode for bulk ablation of tissue may be defined as the amount of energy dissipated as high-density ablative discharge divided by the total energy dissipated by the device. Because the electrode is immersed in a conductive fluid, energy will flow from all uninsulated surfaces in contact with the fluid, although energy flowing from non-edge areas will be at a lower density level and will, therefore, dissipate in the fluid with no desirable effect. This low density discharge can be minimized by insulating the non-edge surfaces from the conducting fluid and/or selecting electrode shapes which minimize non-edge surface areas.
The invention with its tubular structure has been discovered suitable for large volume tissue ablation in a monopolar mode at lower power levels and with conventional electrosurgical generators, thus enabling electrosurgical ablation with much simpler and less costly monopolar systems.
While other monopolar suction electrodes having a tubular cross-section are known and commonly used in nonarthroscopic surgery, they are suitable only for cutting or coagulation and will not ablate if placed in a fluid-filled arthroscopic environment. Their inability to ablate is the result of the manner in which they are insulated. That is, they generally have a layer of relatively thin polymeric insulation extending to within a specified relatively large distance of the electrode distal tip. The specified distance is such that a large area of the electrode (typically approximately 4 mm in diameter and 4 mm in length) is exposed so as to allow coagulation through contact between the tubular electrode's external circumferential surface and the tissue in the nonarthroscopic environment for which they are designed. Such a cautery would be ineffective in a saline environment due to the large uninsulated surface area as virtually all of the current would diffuse into the saline with no ablative clinical effect.
It is accordingly an object of this invention to produce an electrosurgical tissue ablator suitable for use with conventional electrosurgical generators in a monopolar mode.
It is also an object of this invention to produce a monopolar tissue ablator capable of ablating relatively large volumes of tissue at relatively low power levels.
It is also an object of this invention to produce a monopolar electrode capable of producing high power density levels sufficient for tissue ablation while being driven by relatively low power levels, preferably less than approximately 50 watts.
It is also an object of this invention to produce a monopolar electrode capable of producing tissue ablation within a surgical field filled with conductive fluid.